Phosphonodithioates as pesticides



United States Patent Ofifice- Patented Feb. 21, 1967 This invention relates to organophosphorus esters and in particular to O-alkyl-S-phenyl esters of phosphonodithioic acid wherein the phenyl ring carries a chlorine in the para position and a methyl group in the meta position. The invention also pertains to pesticidal compositions containing as the active component thereof at least one of the aforedescri-bed organophosphorus esters.

It is known in the pesticidal art that certain O-alkyl-S- phenyl phosphonodithioates constitute a group of organic phosphorus esters which are exceptionally effective in controlling pest organisms falling within the lower orders of classification. For instance, in US. Patent 2,988,474 and German Patent 1,138,049, there is disclosed a family of lower O-alkyl-S-phenyl phosphonodithioates in which the phenyl residue is either free of substituents or is provided with a lower alkyl radical. According to the biological data presented in the aforecited US. patent, the compounds proposed therein, even in minute amounts, were shown to give excellent control of such typical pests as the housefly, American roach, milkweed bug, confused flour beetle, two-spotted mite, and the like.

Another family of O-alkyl-S-phenyl phosphonodithioates having unusually high pesticidal activity is com posed of members identified by the presence of a chlorine atom afiixed to the phenyl group. A typical representative of the series, S-phenyl-chlorophenyl-O-ethyl ethylphosphonodithioate is capable of giving complete control of the salt-marsh caterpillar in the remarkably low concentration of 0.0005 For a more detailed account of these exceedingly virulent organophosphorus esters, reference is made to the previously cited German patent.

Although the O-alkyl-S-phenyl phosphonodithioates as above described represent a decided advance in the pesticidal art, it has hitherto been diflicult to realize the full potential of these compounds because of their appreciable mammalian toxicity. As a consequence, a considerable degree of care is required in the handling and application of such toxicants, a factor which has mitigated against their full utilization in large scale agricultural operations.

It is, therefore, a principal object of this invention to provide a series of O-alkyl-S-phenyl phosphonodithioate esters having a low level of mammalian toxicity while at the same time retaining the high pesticidal activity normally associated with this category of organophosphorus esters. Another object is to provide pesticidal compositions based on the new toxicants of the invention. Other objects and purposes will become apparent subsequently.

In accordance with the present invention, it has been discovered that the aforeenumerated objects can be realized by providing an O-alkyl-S-phenyl alkylph-osphonodithioate in which the phenyl ring contains a chlorine atom flanked on either side by a methyl radical. The chemical configuration can best be summarized by reference to the following general formula:

CIIa

wherein R is a lower alkyl radical such as methyl, ethyl, n-propyl, isopropyl, sec.-butyl, n-butyl, tert.-butyl, and the like and R is lower alkyl as above for R and in addition can be chlorinated lower alkyl such as monochloromethyl, 2-monochloroethyl, and the like. Exemplary structures falling within the ambit of the general formula include the following specific structures:

As can be observed from an inspection of the general formula, the compounds of the invention are structural crosses or hybrids between the alkyl phenyl phosphonodithioates and chlorophenyl phosphonodithioates set forth in the previously referred to US. and German patents. Yet, for reasons which we have not as yet been able to ascertain or account for, such chemical blending of the prior art structures in a single configuration results in greatly diminished mammalian toxicity while at the same time not significantly aifecting the pesticidal activity of the compounds. In this connection, reference is made to the table showing various comparative test results using the toxicants of the invention and typical representatives of the prior art. Attention is particularly directed to Compound 1 having a mammalian toxicity of 63 whereas the corresponding chlorophenyl exhibited a toxicity of 5.7, the alkylphenyl, a toxicity of 16, the unsubstituted phenyl, also a toxicity of 16. It can thus be seen that the mammalian toxicity of the compounds of the invention are anywhere from one-fourth to one-tenth those of the hitherto known O-alkyl-S-phenyl alkylphosphonodithioates. Yet, as the table clearly shows, the pesticidal activity of our new toxicants against such typical organisms as the housefly, American roach and salt-marsh caterpillar are no lower and in fact are comparable with the known related structures but which have high mammalian toxicity. It is therefore possible to use the compounds of the invention in large scale agricultural and farming operations with much less concern that they will cause injuries to livestock or other valuable mammals.

All. =Amcrican Roach.

S.M.C. =Salt-Marsh Caterpillar.

It is also interesting to note that the position of the methyl and chlorine substituents is quite critical and must be affixed at the position on the phenyl ring indicated in the formula, i.e., a para chlorine atom flanked on either side by a methyl radical. To illustrate the singular specificity of the compounds of the invention, we prepared various isomers such as, for instance, a para chlorine on the phenyl ring while the methyl was affixed one position removed from the chlorine, i.e., meta to the chlorine. The resulting structure was many times less active and was almost devoid of activity in pesticidal quantities. In fact, it exhibited no action against the American roach.

The compounds of the present invention are prepared by reacting the requisite 4-chloro-3-methyl-benzenethiol with the required O-alkyl alkylphosphonochloridothioate in basic media. Typically, the components are refluxed in a relatively inert, normally liquid organic solvent in the presence of an organic base such as triethylamine. Solvents which have proved especially suitable for carrying out the reaction are the normally liquid aromatic hydrocarbons as exemplified by benzene, toluene, xylene, mesitylene, and the like. The products tend to be yellow, viscous oils soluble in the usual organic solvents but insoluble in water.

Reference is now made to the following examples which are inserted for illustrative purposes only. It is to be pointed out, however, that different modifications in practicing the invention will be evident to those skilled in the art without departing from the scope or spirit of the invention.

Example 1 .O-etl1yl-S-( 3-111etlzyl-4-clzloroplzenyl) etlzylplzosphonoditlzioate To a mixture consisting of 15.9 g. (0.1 M) of 4-chloro- 3-methylbenzenethiol, 17.3 g. (0.1 M) of O-ethyl ethylphosphonochloridothioate dissolved in 80 ml. of benzene was added 10.1 g. (0.1 M) of triethylamine. Immediately following introduction of the base, the amine hydrochloride began to separate. The mixture was refluxed for one hour after which the triethylamine hydrochloride was filtered off and the filtrate washed with 2% sodium hydroxide solution, followed by a water Wash, and drying the organic layer with anhydrous magnesium sulfate. The solution was then subjected to vacuum distillation and after stripping off the solvent and volatile components, the product boiling at 150 C./0.7 min. was collected. The yield of a yellow oil was 25.3 g. and the yield amounted to 87%. The density of the purified product was 1.241 at 28.5 C. and the refractive index 1.5903 at 26 C. The results of chemical and instrumental analyscs confirmed the above depicted structur Example 2.-O-metl1yl-S-(3411ell1yl-4-clzl0r0phenyl)- etlzylphosplzonodilhioate Gino The preparation as set forth in Example 1 was repeated but using O-methyl ethylphosphonochlorid-othioate in lieu of the corresponding O-ethyl derivative. The product in this instance was obtained in the form of a yellow oil having an N of 1.6094. The yield was Example 3.-0-pr0pyl-S-(3-112eIlzyI-4-chlol'oplzcllyl) e1hylphosphonodithioate The procedure as given in Example 1 was again can ricd out but using O-n-propyl ethylphosphonochloridothioate as the organophosphorus reactant. The product in this instance was obtained in a yield of 88% and was an essentially colorless oil having an N of 1.5738.

Example 4.-O-ethyl-S- (2-12161hyl-4-clzlorophcnyl) etlzylphosphonodit/ziozltfl C [-13 CzI-Is S The procedure was followed as given in the previous example but using 4-chloro-2-methylbenzenethiol and O-' ethyl ethylphosphonochloridothioate as the organophos phorus intermediate. The purpose in preparing this compound was to demonstrate the cr'iticality of the posi tion of the methyl and chlorine substituents on the phenyl ring. As can be seen from the table, Example 4, in which the methyl and chlorine were situated meta to each other, was relatively inactive as a pesticide.

As those in the art are well aware, various techniques are available for incorporating the active component or toxicant in pesticidal compositions. Thus, it may be convenient to prepare a pesticide in the form of a liquid or solid, the latter preferably in the form of homogeneous free-flowing dusts formed by admixing the active component with finely divided solids or carriers as exemplified by talc, natural clays, diatomaceous earth, various flours such as walnut shell, wheat, soya bean, cotton seed and so forth. Liquid compositions are also useful and usually comprise a dispersion of the toxicant in a liquid media. For instance, it may be desirable to dissolve the toxicant directly in a solvent such as kerosene, fuel oil, xylene, alkylated naphthalenes or the like and apply such organic solutions directly to a pest habitat. However, it is more common practice to employ a dispel'sion of the toxicant in an aqueous media and such compositions may be produced by forming a concentrated solution of the toxicant in a suitable organic solvent followed by dispersion in water, usually with the aid of surface active agents. The latter, which may be of the anionic, cationic or nonio-nic types, are exemplified by sodium stearate, potassium oleate and other alkali metal soaps; detergents such as sodium lauryl sulfate, sodium naphthalene sulfonate, sodium alkyl naphthalene sulfonate, methyl cellulose, polyoxyethylene, fatty alcohol ethers, polyglycol fatty acid esters and other polyoxyethylene sulface active agents.

The proportion of these agents commonly comprises 115% by weight of the pesticidal compositions although the proportion is not critical and may be varied to suit any particular situation. Other adjuncts may be resorted to in compounding biocidal formulations, and in this connection, reference is made to adhesives, spreaders, activators, fertilizers and the like.

The particular method for obtaining the pesticidal data as summarized in the table was carried out as follows:

Test Species:

American cockroach Periplaneta ameri cana (Linn.) House flyMusca domestica (Linn.) Salt-marsh caterpillarEstigmene acrea (Drury) Methods and materials Adult house flies and nymphs of the American cockroach are caged in cardboard mailing tubes with cellophane bottoms and coarse mesh nylon tops and supplied with food and water. From ten to twenty-five insects are employed per cage, depending on the species.

Candidate materials are dissolved in m1. of a suitable solvent, usually acetone. Aliquots of the toxicant solutions are suspended in water containing 0.0175% v./v. Sponto 221, an emulsifying agent, and sprayed on the caged insects. All compounds are screened initially at 0.1% by spraying with a DeVilbiss hand sprayer at p.s.i. in a fume hood. Final mortality readings are taken after 72 hours.

The salt-marsh caterpillar is screened in the following manner: Dock leaves, approximately five inches long, are dipped in aqueous suspensions of the test materials for ten seconds. The leaves are then placed in one-pint food containers with the petioles projecting through small holes in the bottoms into vials containing tap water. Five third instar (910 days old) salt-marsh larvae are introduced and the container closed with a Petri dish lid. Mortality readings are taken at twenty-four land forty-eight hours.

The mammalian toxicity was determined using albino rats and the results are reported in milligrams of toxicant per kilogram of body weight required to produce mortality when administered orally.

We claim: 1. A compound of the formula:

wherein R and R, are lower alkyl.

2. A compound of the formula:

CaHrO 3.A compound of the formula:

CH3 C2115 S l \LSQCI CHaO 4. A compound of the formula:

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 3/ 1953 Austria.

CHARLES B. PARKER, Primary Examiner.

JULIAN S. LEVITT, Examiner.

GEORGE A. MENTIS, FRANK M. SIKORA,

RICHARD L. RAYMOND, Assistant Examiners. 

1. A COMPOUND OF THE FORMULA: 